This application claims the benefit of priority under 35 U.S.C. xc2xa7 119(a) of Korean Patent Application No. 02-18249, filed on Apr. 3, 2002, the contents of which are hereby incorporated by reference in their entirety.
1. Field of the Invention
The present invention relates to the field of semiconductor devices, and more particularly, to an output driver circuit in a semiconductor device.
2. Description of the Related Art
An output driver circuit in a semiconductor device (e.g., an integrated circuit) is used to output a signal representing an internal data state of the device through an output terminal, for example, through an output pad. One type of output driver employed in semiconductor device is a push-pull driver. Output push-pull drivers are used to control the voltage level of the output signal by setting a driving strength, and to control the slew rate of the output signal.
The driving strength of an output signal represents a strength at which the output signal is driven toward the voltage level of a particular source voltage (e.g., a power supply voltage or a ground voltage). Specifically, the driving strength determines how close to the voltage level of the particular source voltage the output signal will be driven. As a driving strength increases, the push-pull driver drives the voltage level of the output signal more closely to the voltage level of the source voltage.
The driving strength of an output push-pull driver is defined in terms of a pull-up driving strength and a pull-down driving strength. The pull-up driving strength refers to the strength at which the voltage level of an output signal is driven toward a voltage level corresponding to a xe2x80x9chighxe2x80x9d logic level, or data state, while the pull-down driving strength refers to the strength at which the voltage level of an output signal is driven toward a voltage level corresponding to a xe2x80x9clowxe2x80x9d logic level. For example, the voltage levels corresponding to the high and low data states may be those of a power supply voltage and a ground voltage, respectively.
A slew rate of an output driving circuit represents how quickly the voltage level of an output signal changes from one data state to the other. The slew rate can be understood as a slope of an output signal""s voltage level with respect to time. An up-slew rate corresponds to the slope of an output signal switching from a low level to a high level, while a down-slew rate corresponds to the slope of an output signal switching from a high level to a low level. Higher slew rates are indicated by steeper the slopes. That is, when the slew rate increases, the voltage level of an output signal changes within a shorter period of time.
A high slew rate is advantageous for an output push-pull driver in terms of data skew, but disadvantageous in terms of noise. Lowering the slew rate of an output push-pull driver decreases noise associated with the output signal, but increases data skew.
In addition, the output terminals of several semiconductor devices (e.g., memory chips) may be connected to a single data bus (e.g., a memory bus). In such situations, the output data currently being transmitted over the bus by one of the semiconductor devices may be adversely affected by a non-transmitting semiconductor device, based on the last voltage level at which the non-transmitting device""s output signal was driven.
The present invention is directed to an output driver circuit in a semiconductor device including a push-pull driver, in which the operation of driving the voltage level of an output signal to a xe2x80x9chighxe2x80x9d voltage level is controlled independently from the operation of driving the voltage level of the output signal to a xe2x80x9clowxe2x80x9d voltage level. Specifically, the push-pull driver includes one or more pull-up units for driving the signal toward a first voltage level corresponding to a high data state, and one or more pull-down units for driving the signal towards a second voltage level corresponding to a low data state. The pull-up units are controlled by one or more pull-up control signals, while the pull-down units are controlled by one or more pull-down control signals.
In an exemplary embodiment, the pull-up control signals include one or more up-driving strength control signals, and the pull-down control signals include one or more down-driving strength control signals. The up-driving strength control signals control the pull-up strength at which the pull-up units drive the output signal. Accordingly, the up-driving strength control signals allow an output signal corresponding to a high data state to be set at a desired voltage level. Similarly, the down-driving strength control signals set the desired voltage level of an output signal corresponding to a low data state, by controlling the pull-down strength for the pull-down units.
According to a further exemplary embodiment, the pull-up control signals include an up-slew rate control signal, and the pull-down control signals include a down-slew rate control signal. The up-slew rate control signal controls the operation of the pull-up units in order to set the up-slew rate of the output signal to a desired rate. The down-slew control signal controls the operation of the pull-down units in order to set the down-slew rate of the output signal to a desired rate. Therefore, the up-slew rate and down-slew rate of the push-pull driver can be independently controlled.
In a further exemplary embodiment, the output driver of the present invention may enter a xe2x80x9ctri-statexe2x80x9d when the semiconductor chip is not currently outputting data. Accordingly, an output enable signal is used to enable the output driver to generate an output signal, or to set the output driver in a tri-state.
Specifically, when the semiconductor chip outputs data, the output enable signal is activated and the output driver generates an output signal whose voltage level corresponds to the internal data state of the semiconductor chip. When the output enable signal is not activated, neither the set of pull-up units nor the set of pull-down units drive the output signal toward the first or second voltage level, respectively. Accordingly, the output driver generates an output signal whose voltage level corresponds to the tri-state. In other words, the voltage level of the output signal is sufficiently lower than the level corresponding to the high data state, and sufficiently higher than the level corresponding to a low data state, thus indicating a xe2x80x9cno dataxe2x80x9d state.
In a further exemplary embodiment, the output driver is controlled to enter into, and exit from, the tri-state in synchronization with the generation of output data by the semiconductor device, based on an internal clock signal of the semiconductor device. Such synchronization helps prevent a situation in which the output driver departs the tri-state before the output data is prepared, thereby causing the semiconductor device to output erroneous data. Also, a situation can be avoided where the output driver exits the tri-state too late, causing the semiconductor device to output data after an expected time.